How to fix bland-looking renders (Cycles)
In short, your renders look plain because you're mostly using out-of-the-box settings.


You should experiment with creating more complex materials. It appears you are currently using mostly plain Diffuse with some simple glass for the table. Break out that Node Editor and tweak the noodles!
Real world materials are some combination of diffuse and glossy, not either/or. Many materials such as plastics, marble, and skin also have Subsurface Scattering. There is a lot you can study about materials, so I'll leave it at this for a start.


Diffuse only:



SSS / Glossy / Diffuse mix:




Try lighting with HDRi panoramas, like PGMath suggested. You can start by browsing here. When using environment lighting, it's best to enable Multiple Importance in the World context, and maybe boost it to 512 or 1024. Use .hdr files instead of .jpg whenever possible because they contain more color information. Better lighting in general will always help your scenes. Lighting is another a vast topic worth studying. Learning how to light with HDR images will take you far.


Lit with a single-color sky:



Lit with a single-color sky plus an emitter mesh:



Lit with an HDRi sky:




You could add some bevel to the edges of your geometry. In the real world edges are not perfectly sharp. Beveled edges catch light and add to the realism. Not sure if this goes against the low-poly style you're using, but it's something to try.


No bevel:



Bevel (with smooth shading):




Add some texture, even if it's subtle, and even if it's just procedural. 
You can use your geometry to automatically create color variation, for example by using Pointiness output from the Geometry Node.


Pointiness can change the color where edges are at steeper angles:



Pointiness node setup:




Add some bumps to your materials. Small imperfections can add up to create lots of juicy details. Glossy maps or other types of maps that factor the mixing of two different types of materials are also very effective. The real world is full of little nicks, scratches, bumps, and cracks, so we expect to see such imperfections.


Before bump displacement:



After bump displacement:




Depth of Field can really enhance your renders and give realism. So can motion blur if you're making animations. Blur can be a good thing.
Fog is a great way to add depth to large scenes. Just don't overdo it.
Play with particles when appropriate. While the settings can be intimidating, particles are a good way to add lots of detail that would otherwise be impossible to create piece by piece.
Don't forget the Compositor. There are many color effects and other enhancements that are possible via compositing.


I may add more example images later. For now those are some ideas.